I was disappointed how unsophisticated the electronics was in model rail, but like so many real-world situations there is so much legacy kit investment out there you can't innnovate overnight. Every newcomer confidently attempting to sweep away the troubles of past systems completely underestimates the challenges of power distribution, control technique and packaging of the circuits. Model rail as a whole is its own universe, with only passing similarities to other fields.
So at the same time I have to be impressed with how much modernisation is getting done, much by hobbyists with no engineering background.

Way to go, Glen! That's the spirit that makes American ingenuity world famous. As a lifetime follower of model railroading I contend that my study of Dad's cabinets full of Model Railroader magazines is one of the primary reasons I'm really good with complex embedded computer systems today...

Yes, pulse width modulation in various forms is almost universal today thanks to the transistor. It makes a small motor develop a higher low speed torque than pure DC, very useful when performing operations such as realistic yard shunting or spotting freight cars on a siding. It can also overheat a motor if the voltage is too high with the duty cycle low (I^2R with very little back EMF), so the pulse peak must be kept within the limit of the motor full speed DC voltage rating. Many a Z scale motor has smoked at low speed when used with a PWM power pack intended for HO scale.
In the article pulse width modulation was used but did not help the main problem which was the opening of the current flow to the motor. This was simply dirt and poor design.
One of the fun things about model rail is combining the hobby with some of the cool things that can be done with electronics and mechanics. Sometimes after a hard day's debugging it is relaxing to come home, pick up a soldering iron, and build a gadget that flashes LEDs at a grade crossing. For examples see
Model Railroad & Misc. Electronics
http://home.cogeco.ca/~rpaisley4/CircuitIndex.html

Aluminum develops a non-conductive surface oxide which will insulate current flow to the wheels. Model rail tracks today are almost always made of nickel silver (actually copper nickel zinc alloy) whose oxide is electrically conductive. Your switch cleaning oil probably retarded the buildup of oxide.
It sounds like you had a lot of fun with the conversions. Nice when folks can use their engineering skills for "play". Not many careers provide that option.

I remember from many years ago, a quick Google search shows December 1965, a Popular Electronics article on a pulse power supply for model trains.
I believe it used a constant pulse (width and amplitude) plus a variable DC to allow smooth control at slower speeds. Even larger gauge trains run poorly at the slowest speeds, but
the article claimed that pulse power allowed slow operation.

This article takes me back to year1992. I converted LEO hand pulled toy train engine compartments into electric train by fixing them with casette player motors,capstan shafts and pulleys.The metal wheels were from steei made in a lathe. The tracks were made from aluminium U channels(inverted) with wooden repers. Connect them with a battery adopter it use to run nice for some time and stops later due to the carbon deposited on the tracks and wheels. I got the idea of applying switch cleaning oil on the tracks and tested and found that it runs continiously for hours together.all the children around played the train and it was amusing!